1. Field of the Invention
The invention relates to a process for producing fibrous material from lignocellulosic raw materials with little use of chemicals. The invention relates in particular to a process for producing fibrous material having a high lignin content, e.g., of more than 15% for softwoods and more than 12% for hardwoods, based in each case on the oven-dry fibrous material produced, the fibrous material generally having specified strength properties.
2. Background Information
Processes which produce fibrous materials having a relatively high lignin content of more than 15% for softwood and of more than 12% for hardwood are known. They give a yield of 70% or more, based on the starting material used. These processes are based on chemical and/or on mechanical disintegration of the wood.
In the case of mechanical defibration of the wood, the latter is separated into fiber bundles in beating units—generally after presteaming. These fiber bundles are then defibrated into individual fibers by further beating. The yield is very high but so is the quantity of beating energy to be applied. The strength of the wood fibers is very low—even after beating—because the fibers contain a large amount of natural lignin and therefore have little binding potential. They are also strongly degraded by the mechanical defibration, which adversely affects their recyclability.
In the case of chemical digestion of the wood, chemicals act on the wood, generally under high pressure and at elevated temperature. The NSSC process may be mentioned as a typical process for high-yield fibrous materials (Semichemical Pulping for Corrugating Grades, page 130 et seq.; in Pulp and Paper Manufacture, 3rd Edition, Vol. 4, Sulfite Science and Technology—ISBN 0-919893-04-X). In this process, an alkaline component, typically sodium carbonate, is also always used in the digestion solution in addition to sodium sulphite. However, other processes, such as the kraft or the soda process, can also be modified so that high-yield fibrous materials are produced (cf. “Choosing the best brightening process”, N. Liebergott and T. Joachimides, Pulp & Paper Canada, Vol. 80, No. 12, December 1979). If the degradation of the originally used wood is to be limited to a maximum of 30%, far less chemicals are required and used than in the production of chemical pulps which are to be completely freed from lignin. For the production of high-yield chemical pulps, the amount of chemicals is metered as a function of the desired yield. In order to achieve a yield of about 70%, based on oven-dry wood use, the prior art recommends using up to 10% of chemicals, based on the starting material. In the case of chemical pulps, the use of chemicals is often 30% or more of chemicals, based on the oven-dry wood.
Chemicals play a role in determining the process costs, i.e., they are used as sparingly as possible. CTMP fibrous materials are usually produced with amounts of 3% to 5% of chemicals. In known, industrially established processes for producing high-yield fibrous materials, e.g., the NSSC process, up to 10% of chemicals, based on the starting material, are used. With the use of chemicals limited in this manner, no recovery is as yet installed for recovering the chemicals. In spite of the relatively small amounts of chemicals, this method of fibrous material production leads to considerable pollution of the environment, in particular of bodies of water, not only because of the introduction of chemicals but especially because of the organic load which is released into the main outfall.
In addition, sharply rising energy prices add to production costs in the case of mechanically produced fibrous materials. In the case of chemically produced high-yield fibrous materials, the production is adversely affected by the costs for the lost chemicals.
High-yield fibrous materials are beaten to high freenesses for the current intended uses. Only then do they reach an acceptable strength level. Here, high freenesses are to be regarded as values of about 300 ml CSF (Canadian Standard Freeness), equivalent to 41°SR (Schopper-Riegler, see below) and 500 ml CSF, equivalent to 26°SR, as described, for example, in “Choosing the best brightening process”, N. Liebergott and T. Joachimides, Pulp & Paper Canada, Vol. 80, No. 12, December 1979, for high-yield fibrous material from softwood. A high freeness is achieved by using mechanical energy. The fibers are rubbed against one another or against a grinder or against a grinding medium and thus changed in their surface properties to achieve better binding behavior. The high freeness is therefore not an end in itself. Rather, it arises out of the requirements regarding the strength properties of the fibers.
The high-yield fibrous materials produced by a mechanical and/or chemical method are used in particular where high final whiteness and high whiteness stability are not absolutely essential. They could open up numerous further fields of use if the strength level could be increased.